Delta q-meter



Dec. 28, 1943. w. o. LOUGHLIN DELTA Q-METER Filed Aug. 15, 1941 9 m my 1 g MA 5 .5. f r 3 w V g 69 v m a m ,0 m J 2 u 0:06 P x) .n

Q FACTOR FREfiLENCY REACTANCE FACTOR v INVENTOR ififlmm D. L ol gipZz/iz/ ATTORNEY Patented Dec. 28, 1943 2,337,159 V DELTA Q-METER William D. Loughlin, Mountain Lakes, N. .r;, assignor to Boonton Radio Corporation, Parsinpany-Troy Hills Township, Morris County, N. J.

Application August 15, v19 ;1, Serial Nc. 408,981

' -s Claims. (Cl.1'l5-183) The present invention concerns electrical apparatus and in particular a device for measuringthe Q or loss-factor oi a condenser, inductance or oscillatory circuit.

One object o! the present invention is to provide an improved Q-meter.

Another object is to provide a Q-meter operat ing in a simplified manner.

still another object is to provide a Q-meter' which measures Q values directly 01 eith 9. capacity or an inductance.

A further object is to provide a Q-meter which measures the Q of condensers, inductances and circuits of combined capacity and inductance directly and which is particularly adapted to make such measurements at high frequencies, for instance, at frequencies above 30 megacycles.

These and other objects will beapparent from the detailed description of the invention given in connection with the various figures or. the drawing. In the drawing:' I

Figure 1 shows a circuit present invention.

Figure 2 shows a condenser dial for one form I 01 the present invention.

Figure 3 shows a Q-factor scale for one form of the present invention. v

Figure 4 shows a frequency diali'or one form of the present invention. 'f

Figure 5 shows a modified circuit'of one form of the present invention.

In the past a common method of measuring the of a coil has been to measure the voltage step-up in a tuned circuit formed with the coil. A small known voltage from a convenient source, such as a thermionic vacuum tube oscillator, is placed in series with the coil to be measured. The coil is resonated by means of a variable condenser and the resulting voltage across the coil is measured. For appreciable values of Q the Q is the ratiov of the voltage across the coil to the voltage in series with the coil. A Q-meter according to the above description is shown and described in detail in Patent Number 2,137,787, entitled Method and apparatus for electrical measurements" and which was issued to Harold A. Snow on November 22, 1938. In this Q-meter the voltage placed in series with th coil is kept con- 61 one roam or the.

stant and the voltmeter connected across the coil is calibrated directly in terms of Q. The Q of a condenser may be measured by placing it in parallel. with the resonated coil and returning the variable condenser to produce resonance. The Q of the parallel condenser is given by the Iormulm; I

where 01 is the initial capacity of the variable i.. condenser, Qi is the initial reading of the Q- meter, C2 is the capacity of the variable con-' denser after connecting the condenser being, measured, and Q: is the Q-meter reading with the condenser to be measured connected in the circuit.- Thus the Q-meter which has been de-- scribed measures the Q of a coil directly but the Q of a condenser indirectly.

The present invention concerns a device for f measuring directly the Q of a circuit having either apositive or negative apparent reactance or no apparent'reactance. This is particularly useful in measuring parallel tuned circuits at very high frequencies. A variable frequency oscillator supplies a known voltage in series with an internal inductance inthe Q measuring circuit.- The inductance of the Q measuring circuit is initially tuned by a constant capacity. The

voltage. acros the Q measuring circuit in the absence of an external circuit is set to infinity on a the Q scale oi the Q circuit voltmeter. In order to change the frequency at which a measurement is to be made the inductance of the oscillator circuit and the inductance of the Q measuring circuit are changed by equal amounts, thereby keeping the Q circuit tuned to resonance with the same capacity. This initial Q circuit capacity is marked zero on its control dial. It the Qof a condenser is to be measured it is connected .in' parallel with the Q measuring circuit and the Q circuit variable condenser is decreased until resonance is reached. The Q-meter dial is read and multiplied by a factor on the condenser dial; 1

to give the true Q of the external condenser. One possible scale for the Q-meter dial is derived from the above formula for Qx assuming Q1=100 and C2=C1 for unity on the capacity dial. That is, s

= act cl-a is marked on the Q-meter dial assuming This gives a scale having infinity at and at mid-scale and may becalled a scale denser dial in the Q measuring circuit are marked in ratios of the external capacity to Cl. as given by the ratio C: Cl

These factors may be positive or-negative to indicate an increase or decrease or capacity to tune an external inductance or capacity. Thus, if the value of C1 is chosen as 100 mmi. the factor on the condenser dial will be -0.1 for a decrease of capacity indicating anexternal capacity of 10 mmf. The condenser dial will show a positive factor for externally connected inductive cir. cuits. Where the external circuit being measured is a parallel tuned circuit or a pure resistance, no change in the condenser dial will be required indicating zero reactance. In order to maintain the calibration of the Q-factor dial as the frequency is varied, means are provided also ganged to the frequency control for maintaining a constant Q in the internal Q circuit as, for instance, a variable resistor in shunt with the circuit.

Figure 1 shows a circuit diagram of one form of the present invention. It includes an oscillator tube I having cathode 2, heated by welllmown means not shown, control grid 3' and plate 4. Tube is associated with a variable inductance tank coil 3 by connecting plate l to one end through condenser 1, grid 3 to the other end through resistor 3 by-passed by condenser 3, and cathode 2 to an intermediate point and to ground 22 by means of ground connection 3. Plate voltage is supplied to plate 4 from a suitable source such as battery through the parallel feed choke ||l. Tank inductance 5 is tuned by fixed condenser 33. The frequency of oscillation of tube l and its associated circuit is determined by the capacity of condenser 33 and the inductance of tank coil 5. The frequency of oscillation is varied by varying the inductance of tank coil 3 as indicated by the arrow. I

High frequency current from the oscillator may be i'edto the Qmeasuring circuit in a number of well known ways as, for instance, by

means of pick-up coil 2 and injection resistors l3 and I4 connected to ground at their junction. It may be convenient to measure the current being fed to resistor l3 and II. If so a thermocouple heater |3 heating junction |3 connected to meter I! may be placed in series with the resistors and pick-up coil l2. Since it is desired tobalancethe current in resistors l3 and I4 to ground a resistor equal to the resistance of heater I8 is placed in the opposite side of the feed circuit. A parallel resonant circuit is formed by connecting variable condensers 2'0 and 2|, having equal and variable capacities to ground, in series and in parallel with variable inductances 34 and 33 connected in series withresistors l3 and I4. The variable 'inductances 34 and 35 are gan ed and are controlled by the same control which varies inductance 3. The

sum of the inductances-of inductors 34' and 33 is equal to inductance 5. Condensers 20 and 2| have a mid-scale value equal to twice capacity 33 so that their series capacity is equal to capacity 33. Thus with condensers 20 and 2| set at mid-scale, circuit |3-|l-3l-35-2||--2| is always :in tune with the frequency of oscillator 2|. Meter 3| and resistor 29 may be by-passed by a condenser 30. The Q of the circuit |3--|4--20-2|-3l--35 will he the ratio of the voltage across condensers 20 and 2| as indicated by meter 3| to the injected voltage across resistors l3 and H as indicated by meter l3.

In order to permit connection of a circuit to be measured, terminals 23, 23 and 24 are provided connected to either side of condensers 23 and 2| and to ground respectively. In order to provide a basis for the Q-factor dial calibration circuit |3|4-2|l-2|3435 is kept at a constant predetermined Q by means of a ganged Q control means such as variable resistor l5 ganged to the control of inductances 5, 34 and 35. Resistor |5 has a law of variation such that as the frequency of oscillator is varied the Q of circuit |3--|l2|l--2|-3|-35 is kept constant.

The Q of an externally connected circuit such as condenser 36 connected across terminals 23 and 25 is given by the formula:

The Q-factor dial, 1. e. the dial or meter 31,15 calibrated to be equal to the factor Hence. the Q of condenser 36 is found directly by multiplying the reading of the Q-factor dial by the reading of the reactance factor dial.

as the frequency isrvaried. The voltage across the resonant circuit is measured with a vacuum tube voltmeter such as diode 28 having plate 23 and cathode 21 connected in series with meter 3| C1 is the capacity value of the Q circuit tuning condenser (2|-22) initially. Qi is the Q with no external circuit connected corresponding to C1, C2'

is the value of the Q circuit capacity required to produce resonance with the circuit to be measured connected, and Q1 is the Q of the entire circuit including the circuit to be measured. A typical reactance factor dial is shown in Figure 2. The initial capacity required to tune the Q circuit is located at the center of the dial and is marked zero. Factorson the decreasing capacity side of zero are marked with a negative sign to indicate external capacities or effective capacities while factors on the increasing capacity side of zero are marked positive to indicate external inductances or effective inductances.

A typical Q-factor'scale is shown in Figure 3 I reactance factor dial will read dial.

Figure 4 shows a dial for the oscillator freand resistor 29 and across condensers 20 afif'l qllen y cont-r01, m n e 5. This dial also is read on the Q-factor dial.

and Q compensating The value C1 is known and f is read on the frequency control dial in megacycles. The value Rp is the resistance in ohms of a resistor or the tuned impedance of a parallel tuned circuit.

Figure shows a modified circuit which is useful for single-ended circuit measurements. In-

ductances 34 and 35 are replaced by the single inductance 31 which is variable and ganged and tracked with oscillator inductance 5. The oscillator voltage is injected by means of coil 40 coupled to oscillator inductance 5 and which feeds resistor 39 in series with the Q circuit.

Condensers 20 and 2i are replaced by the sin gle variable condenser 38, and as before resistor I 5 ganged with the oscillator inductance control maintains constant Q in the measuring circuit. A single-ended parallel-resonant circuit made up of coil 42 tuned by condenser 4| is shown to be connected to terminals 23 and 24 for measuring its Q. Also shown in Figure 5 is provision for measuring large condensers, small inductances or small resistors by series connection with the Q measuring circuit. This series connection is made by connecting the circuit to be measured across terminals 43 and 44 in series with the Q circuit after removing shorting link :35.

When circuits having a low Q are measured it may be useful to increase the injection voltage. This may be done by a definite ratio from a scale on meter l9 and the Q-factor meter reading corrected accordingly. A larger deflection being produced on the Q-factor dial is more accurately readable.

While only two possible forms of the present invention have been shown and described, many modifications will be apparent to those skilled in the art within the spirit and scope of the appended claims.

What is claimed is:

1. In a device for measuring the Q of an impedance, the combination of, a measured source of high frequency current, a parallel tuned circuit including said source in series and initially tuned by a predetermined mean capacity value of a variable condenser, means for connecting an impedance to be measured in parallel with said tuned circuit, a voltmeter across said tuned circuit calibrated in terms of Mr QX Q2 where Qi is the 'Q of the tuned circuit before oi reactance from a predetermined connecting the external circuit and Q2 is the Q of the combined tuned and external circuit, and a scale on said variable condenser in terms of where C1 is said mean capacity and C2-C1 is the apparent reactance of the external circuit, and the Q of the external circuit being Qx where Qx is equal to the voltmeter scale reading times the variable condenser scale reading.

2. In a. Q-measuring device, the combination of, a source of high frequenc alternating current, a circuit tunable to the frequency of said current, means for injecting at least a portion of said current into said circuit, constant capacity means for tuning said circuit to said frequency, single control means for varying the frequency of said source and said tuning means, means for coupling a circuit to be measured with said circuit and means for measuring the voltage across the first said circuit before and after the coupling of said external circuit in terms of a factor which is a function of the Q of the first said circuit and the Q of the coupled circuit.

3. In a Q-measuring device, the combination age across said tuned circuit calibrated in terms of the product of the Q of the tuned circuit and the Q of the circuit to be measured divided by the diiierence of said Qs.

4. In a Q-measuring device, the combination of, a source of high frequency alternating current, a frequency calibrated control for varying the frequency of said source, a tuned circuit coupled to said source of current, a tuning control for said circuit calibrated in ratios of departure value to that value, means for keeping the Q of said tuned circuit substantially constant as said tuning is varied, means for coupling a circuit to be measured to said tuned circuit, and means for indicating the voltage across said tuned circuit calibrated in terms of the product of the Q of the tuned circuit and the Q of the circuit to be measured divided by the diflerence of said Q's.

5. In a Q-measuring device, the combination of, a source of high frequency alternating current, a tuned circuit coupled to said source of current, means for maintaining the Q of said circuit constant, a single control for varying the frequency of said source, the tuning of said circuit and for operating said Q maintaining means, means for coupling a circuit to be measured to the first said circuit and means for indicating the resulting voltage across said tuned circuit for measuring the Q of the said circuit to be measured. 

